The present invention relates to a multi-layer structure for controlling the transmission of laser radiation, and more specifically to a multi-layer structure whose outer layer partially ablates to form an irregular surface when exposed to laser radiation and whose intermediate layer contains a diffractive array of monofilament fibers, the outer and intermediate layers thereby significantly attenuating the on-axis transmission of the incident laser radiation. This invention may be used in such structures as aircraft canopies and windshields, helmets and glasses.
The following co-pending applications by the same Applicant are related to the present application:
1. "Partially Ablating Laser Resistant Structures", Ser. No. 526,602, filed simultaneously with the present application;
2. "Heat Conducting Laser Resistant Structures", Ser. No. 526,603, filed simultaneously with the present application; and
3. "Absorbent Laser Resistant Structure", Ser. No. 526,605, filed simultaneously with the present application.
In recent years there has been considerable interest in and a growing development of high intensity lasers. By optical radiation as used herein is meant any portion of the electro-magnetic spectrum between about 0.4 and about 15.0 micrometers, and is not confined to the visible portion of the spectrum. Already, considerable effort has been directed toward the development of communication systems utilizing lasers while further technical development is being directed toward the use of a laser as a weapon against aircraft.
Many current optical devices are fabricated with glass, glass laminates, plastics and plastic laminates to achieve various effects under sunlight or artificial light conditions. Several of these devices have been designed with the idea that light will be diminished as it passes through the device. Many modern applications require these devices to transmit light in the visible portion of the spectrum but diminish light from high intensity laser radiation.
To date, laser weapons have two practical threat wavelengths:
1) approximately 10 micrometers (infrared; and, PA1 2) the range 3 to 5 micrometers. PA1 a) Aerodynamic compatibility with the shape factor of the aircraft; PA1 b) Structural soundness under prescribed aerodynamic environments; PA1 c) Good visibility (transparency in the visible portion of the spectrum); and PA1 d) Ease of fabrication. PA1 e) be capable of withstanding laser radiation for a sufficient time (e.g. t &gt;1 second) to enable the pilot to assume a change in aircraft attitude; PA1 f) be aerodynamically sound to enable the pilot to carry out his assigned mission and/or return to his home base after having been exposed to some maximum level of radiation; PA1 g) to reduce the on-axis level of radiation transmitted, thereby lowering the level of exposure to the pilot.
Depending upon the type of material exposed to the incident laser beam, the energy from each of these threat wavelengths will either be absorbed by or transmitted through the layer of material. In general, however, plastics will be absorbing to the 10 micrometer laser while some will transmit in the 3-5 micrometer range. Thus, based upon intelligence reports different structures will be employed according to which laser wavelength is anticipated.
One light disruptive device is disclosed in the patent to Horton, U.S. Pat. No. 3,561,842. The structure relies on the heating effect of absorbed high-intensity optical radiation to destroy a film of light transmissive material which is bonded to a roughened substrate. When high intensity optical radiation impinges the film surface, the energy is absorbed and the film is destroyed, exposing the roughened substrate which deflects the beam. The roughened substrate is a source of possible shortcomings in that it presents additional steps in fabrication, difficulties in providing a continuous index of refraction and an increased absorptivity to radiation due to the roughened surface. In addition the roughened substrate is a pre-induced damage control device whereas the present invention concerns a self-induced damage mechanism.
Aircraft canopies and windscreens have generally been designed for aerodynamic and economic reasons rather than for laser weapon protection. They are constructed to incorporate the following characteristics:
However, in view of the developments in laser technology, it is now desirable that aircraft canopies and windscreens also: